1. Field of the Invention
The invention relates to laser machining and more particularly relates to laser machining of crystalline and poly-crystalline materials and to silicon integrated circuit manufacturing and testing.
2. Description of the Prior Art
Lasers have been used for marking and machining of materials since shortly after their invention. Established techniques include laser cutting, drilling, and welding. These processes have been applied to a wide range of materials including, metals, ceramics, polymers, and natural-products such as cotton and paper.
Many types of lasers, including both continuous wave (CW) and pulsed systems, are used for material modification. In general, pulsed systems provide higher peak powers and CW sources provide greater total power. For pulse widths greater than 10 picoseconds the modification process is dependant on the ability of the material to adsorb the light. (Pulse widths are measured at FWHM.) The modification process is therefore wavelength dependant at power levels sufficient to machine materials. Light sources across a large region of the electromagnetic spectrum are, therefore, used for machining. These include CO2 lasers in the infrared and excimer lasers in the ultraviolet. It has been believed that wavelength is less important when pulse widths are shorter than approximately 10 picoseconds.
In contrast, the lengths of laser pulses are known to be important parameters in many instances. Fluence, or energy per unit area, is also a critical factor. Short and powerful pulses produce results markedly different from long and less powerful pulses.
The effects of fluence and pulse length are in part due to the different processes that can occur in laser machining. These include heating and melting, blasting, and plasma development. Heating and melting are the result of thermal effects wherein photon energy is absorbed by the target and converted into thermal energy. A thermally affected zone is one in which material has been heated to the extent that its chemical or electrical functionality are changed. For example, when laser machining occurs near an integrated circuit, the thermally affected zone is the volume within which a state or operation of the integrated circuit is changed or a property that affects a state of the integrated circuit is changed. As the material is heated it melts and a thermally affected zone often extends beyond the area on which light is incident. Blasting is the result of sudden thermal excitation wherein rapid expansion causes material to be blown off of the target. Plasma may also form if the material is heated rapidly. Plasmas imply that electrons have attained sufficient energy to escape from the protons they are normally associated with. Due to the presence of free electrons, energy can travel very rapidly in plasmas. Heating times and cooling mechanisms can also be very important to laser machining processes. The rate at which energy leaves the affected area helps determine the characteristics of the machined material. For example, it is well known that the smoothness of holes drilled in thin metal sheets is much greater for picosecond laser pulses than for nanosecond pulses. Femtosecond laser micro-machining is essentially a non-thermal machining process. Energy is transferred to the material lattice in a picosecond time scale, resulting in a rapid formation of a plasma that expands and expels the vaporized material from the surface.
It is known in the current art that various levels of laser fluence have different effects on a machined surface. FIG. 1 illustrates these regions as expected on a metal surface. In a first region 110 laser pulses have no effect on the material. A second region 130 begins at a damage threshold 120 and continues until a fluence of roughly ten times threshold 140 is reached. In the second region 130 the material is modified but not removed. A third region 150, above ten times threshold 140, is characterized by significant material modification and removal. The difference between the two thresholds 120 and 140 varies widely as a function of the material and fluence of the light. Generally, it is believed that the polarization and wavelength of the light used is unimportant.
Laser machining is most commonly used for metals, ceramics, natural products, and polymers. Other materials such as crystals, polycrystals, or glasses have been described as xe2x80x9cproblem materialsxe2x80x9d in the field. Such materials can have very high melting points or not adsorb photons at the easily produced wavelengths. An example of such a material is silicon. Silicon easily adsorbs light but traditionally forms irregular surfaces when laser machined. Silicon has a damage threshold fluence of roughly 0.13 J/cm2 for 795 nanometer light.
Silicon is a very important material in the electronics industry wherein wafers are used in the manufacture of integrated circuits. There is a significant need for machining silicon with the accuracy and precision required in the integrated circuit industry. Silicon is currently machined using techniques such as grinding or ion beam etching. Grinding is used to remove bulk material but can thermally damage any existing circuits. Ion beam etching on the other hand has the precision and accuracy required but is an extremely slow process.
There are a number of applications that would greatly benefit from improved silicon machining techniques. For example, to test circuits it is advantageous to insert a probe through the backside of the silicon substrate on which the circuit has been built. In current technology ion beam etching is used to drill a hole through the substrate to the circuit to be tested. This process can take many days or even weeks and, therefore, is not practical for frequent use. Lack of a more practical machining process also limits the ability to quickly make connections between integrated circuits on either side of a flat substrate.
FIG. 2 shows a block diagram of a prior art laser machining (or marking) system generally designated 200 and including a laser 210 that produces a light beam 220. Beam 220 is manipulated by a wide variety of optional optics 230 and is finally directed at a target 240.
FIG. 3 illustrates a hole, generally designated 300, drilled using prior art ion beam etching methods. Hole 300 is made in the silicon substrate 310 using a beam of high-energy ions (not shown). The beam slowly sputters material from substrate 310. Typical removal rates are on the order of 100 cubic microns per second. A 500 by 500 by 100 micron hole therefore requires about 3 days at 100 Hz. Hole 300 is made from a backside 312 of substrate 310 to a front side 314. Front side 314 optionally supports various electronic components 320. For example, in FIG. 3, hole 300 is located across from a circuit of interest 330.
FIG. 4 is a micrograph of a hole generally designated 400 laser drilled in silicon using methods of the prior art. The hole has a diameter of about 70 microns. Hole 400 was drilled at a fluence of 4.6 times threshold 120. Bright spots 410 in the center of the hole are peaks of spikes that can rise several microns above the original surface. These are believed to occur as a result of plasma condensation processes. At other fluences pits can occur in the bottom surface of the hole (not shown). These spikes and pits, such as illustrated in FIG. 3, have prevented laser machining from replacing conventional ion beam etching processes in many applications.
FIG. 5 is a cross-sectional illustration of the pertinent features of hole 400 shown in FIG. 4. The hole 400 is made in a silicon substrate 310 on which integrated circuits 320 and 330 have been manufactured. The bright spots 410 (FIG. 4) are shown as spikes 530 and pits 540 are also visible. Spikes 530 and pits 540 prevent further machining without affecting the circuit of interest 330 because the bottom surface 510 of hole 400 has become too irregular.
There therefore exists a significant need in the art of silicon machining for a system and method of micro-machining holes in silicon that produce more desirable surface characteristics, in less time, and without significant peaks and pits.
Systems and methods are disclosed for producing smooth bottomed holes in poly-crystalline materials using laser micro-machining techniques. Surface smoothness of better than 0.5 micron RMS is attained, where smoothness is determined over a 100xc3x97100 micron area. The micro-machining method includes laser pulses whose fluences fall within a region that removes material at an economically viable rate but does not produce significant irregular features such as pits and spikes.
Methods include the use of preferred laser pulse widths, wavelengths, and polarization. One embodiment includes providing a laser system, producing a pulsed laser beam using the laser system, and focusing the laser beam with a focusing optic onto the surface of the material to be modified. The material is supported in a position relative to the focused laser beam, and portions of the material are removed using the pulsed laser beam in a manner such that a surface that is smooth to within 2 microns RMS (root mean squared) is produced, material is removed at an average rate greater than 12 cubic per pulse, and a thermally affected zone penetrates less than 5 microns from the machined surface. In alternative embodiments material is removed at average rates greater than 1 cubic micron per pulse or greater than 6 cubic microns per pulse. These per pulse removal rates are possible at pulse repetition rates equal to or greater than 1000, 5000, 10,000, 50,000, or 100,000 Hz.
Uses of the described for the micro-machining methods include machining silicon substrates used in electronics applications. Systems enabled by the micro-machining methods, and within the scope of the present invention, include devices for probing or modifying circuits through holes made in silicon substrates, electrical connections through substrates, and connected circuit systems occupying both sides of a substrate.
Embodiments of the invention include systems for machining poly-crystalline materials. These systems include a laser system for generating a pulsed laser beam capable of removing material at an average rate greater than 12,000 cubic microns per second, generating thermally affected zones that penetrate less than 10 microns from the machined surfaces, and producing a machined surface smooth to within 5 microns RMS. The laser beam is directed, using a focusing optic, at a support system for positioning the poly-crystalline material in relation to the laser system.